Over 700 million cases of and as many as 5 million deaths from acute gastroenteritis occur each year. Despite intensive investigation, the etiology of most episodes of diarrhea is unknown. Human caliciviruses (HCV) are one of a number of enteric viruses recognized in the last 10 years. Together, these newly discovered agents may cause a majority of diarrheal episodes. HCVs are human representatives of a unique virus family, infect all age groups, and occur worldwide. A paucity of information about HCV reflects in part the difficulties facing investigators seeking to study this virus family. The specific aims of the research proposed here are intended to fulfill long-term objectives to improve understanding of HCV as a family and the role of HCV in human health, to understand the relationship of HCV to other enteric small round viruses, and to increase the availability of reagents for work with HCV. The specific aims of the proposed research are: 1. To produce cDNA clones from the HCV genome. 2. To determine the molecular organization of the HCV genome by sequencing these cDNAs. 3. To utilize the cDNA probes to analyze the genetic relatedness of HCV strains to animal caliciviruses. 4. To utilize the cDNA probes to improve our understanding of HCV epidemiology and the pathogenesis of HCV infection. cDNA clones will be produced in a lambda gtll derivative utilizing RNA extracted from HCV purified from stool specimens. Clones will be screened utilizing our hyperimmune anti-HCV antisera. HCV-specific clones will be selected and sequenced, generating information about viral genome open reading frames, organization, cleavage sites, and regulatory sites. That information will be used to express viral genes in the laboratory, produce monospecific antisera, and characterize the relationship of HCV to other enteric viruses. The cDNA clones and derived reagents will also be used to expand ongoing epidemiologic studies of HCV and to understand mechanisms of HCV pathogenesis.